This invention relates generally to a document reproduction machine and, more particularly, concerns a discrimination system for automatically sensing the position, size, and background density on an original document being copied.
In prior art document imaging systems, it is known to determine various features relating to the original document so that appropriate adjustments may be made to xerographic stations in the overall system. For example, when a document is placed on a platen and incrementally scanned by an illumination source/mirror assembly, it is important to know the position, or lead edge of the document so that the copy media being moved into the transfer zone will be properly registered. For systems utilizing a plurality of paper trays containing sheets of different sizes, it is also important for the system to detect the dimensions of the original document so that the proper copy tray can be activated. Further, document size is important for systems requiring document edge and side erase further downstream of the exposure station. A third function required for many systems is the ability to detect the background density of documents having a background other than white so that appropriate adjustments can be made to the image development system. All of these functions have been implemented in the prior art by devices which sense either singly or in some combination, the particular document characteristics and, which generate signals which are sent to the appropriate xerographic stations. Some examples of representative prior art patents are as follows:
U.S. Pat. No. 4,456,372 to Yamauchi, discloses the use of a transparent glass table and a document detecting member for detecting document density and size of the document automatically once the platen cover is lowered. U.S. Pat. No. 4,541,713 to Maekawa discloses a platen cover with various different colored portions used in conjunction with a light receiver for determining document size. U.S. Pat. No. 4,620,781 to Miyamoto, discloses the use of a black platen cover for determining document size and absorbing reflected light. U.S. Pat. No. 4,749,872 to Asada et al. discloses the use of a CCD sensor array and mirror combination that senses document image and a reference marker for determining document location.
None of the above references discloses a sensing system which simultaneously enables all these functions; document lead edge (position), size, and document background density. To perform the lead edge and document size function, it is necessary to provide adequate optical contrast between the usually white document and the overlying platen cover. This can be accomplished by utilizing a colored platen cover and an appropriate optica filter to limit the sensor response. However, this requirement is incompatible with that of document background sensing since, for this function, it is desirable that the response of the CCD be similar to the photoreceptor response (e.g., it should sense the same light that is being projected on the photoreceptor).
According to the principles of the present invention, the three functions described above are enabled by using an array detector such as a charge-coupled device (CCD) sensor used in conjunction with a glass having an interference coating or a spectrally selective filter overlying the glass filter save for one "window" area. With this combination, the major portion of this sensor detects the scanned image through the wavelength portion of the color chosen to be transmitted through the interference filter. That portion of the sensor generates the signals which represent document position and size. The transparent window portion receives unfiltered light at that portion of the sensor and generates a second signal representing document background density. More particularly, the invention relates to a document reproduction machine for copying original documents placed on a transparent platen, the combination including means for scan/illuminating said document and for projecting a latent image of said scanned document on a photosensitive surface, sensing means positioned on the image side of said projecting means for discriminating between the document being copied on the platen and the area surrounding the document and for generating signals representing document size and position on said platen, said sensing means further adapted to sense the background density of said document and to generate electrical signals representative of said background density, and control means to digitize said sensing means output signals and to generate signals to adjust the operation of a plurality of xerographic stations.